1. Field of the Invention
The present invention relates to a dispersion-type liquid crystal electro-optical device comprising a liquid crystal/resin composite comprising a resin having dispersed therein a liquid crystal material or a resin/liquid crystal composite comprising a liquid crystal material having dispersed therein a resin.
2. Prior Art
Liquid crystal electro-optical devices well known and already put to practice heretofore are those operating in TN (twisted nematic) mode or STN (super twisted nematic) mode, in which nematic liquid crystal compositions are used. Recently, liquid crystal electro-optical devices taking advantage of ferroelectric liquid crystals are also realized. A liquid crystal electro-optical device of the type above basically comprises a first and a second substrate each having provided thereon an electrode and a lead, and a liquid crystal composition having incorporated therebetween. Thus, the liquid crystal composition can undergo a transition between states by applying thereto an electric field through the electrodes provided on the substrates. These changes in states are ascribed to the anisotropy of the dielectric constant of the liquid crystal composition itself in the case of nematic liquid crystals, etc., and to the spontaneous polarization in the case of ferroelectric liquid crystals. In this manner, the electro-optical effect due to the changes in state of the liquid crystal molecules can be utilized to give an electro-optical device.
In the TN mode or the STN mode liquid crystal electro-optical devices, the liquid crystal molecules within the plane of the liquid crystal layer in contact with the substrate arrange themselves along the rubbing direction upon applying a rubbing treatment to establish a molecular orientation. The upper and the lower substrates are displaced from each other in such a manner that the rubbing direction of one substrate make an angle in the range of from 90.degree. or from 200.degree. to 290.degree. to that of the other. Thus, at the central portion of the liquid crystal layer, the liquid crystal molecules are arranged helically to minimize the energy between the upper and the lower liquid crystals which are positioned with respect to each other within an angle in the range of from 90.degree. to 290.degree.. Furthermore, in such a construction, the liquid crystal material in an STN mode device may be a mixture with chiral substances if necessary.
In the conventional type of electro-optical devices as described in the foregoing, however, it is requisite to incorporate polarizer sheets and also to maintain the liquid crystal molecules in a regularly oriented manner within the liquid crystal electro-optical device. The treatment for establishing a molecular orientation comprises rubbing the orientation film (which is an organic film in general) with a cotton cloth or a velvet cloth. If no such treatment is applied, the electro-optical effect of the liquid crystals cannot be expected because no uni-direction oriented liquid crystal molecules would be realized. Accordingly, the device inevitably comprises a pair of electrodes to define a space to maintain therein the liquid crystal material. Thus, the liquid crystal is injected into said space and then subjected to orientation treatment to realize an optical effect.
In contrast to the liquid crystal electro-optical device mentioned hereinbefore, there is also known a dispersion-type liquid crystal which can be employed free of such polarizers and rubbing treatment, and which yet provides an image plane having a brighter contrast. The light control layer of this dispersion-type liquid crystal comprises a light-transmitting solid polymer maintaining therein the liquid crystal material in droplets or in a sponge-like structure. The liquid crystal device can be fabricated by dispersing an encapsulated liquid crystal material into a polymer, and then providing said polymer on a substrate as a film or a thin film. The liquid crystal can be encapsulated with gum arabic, poly (vinyl alcohol), gelatin, and the like.
In a dispersion-type liquid crystal comprising liquid crystal molecules encapsulated with poly (vinyl alcohol) and having a positive dielectric anisotropy, for example, the liquid crystal molecules arrange themselves in such a manner that the major axes thereof become parallel to the direction of the electric field. If the refraction index of the solid polymer is equivalent to that of the arranged liquid crystal upon application of the electric field, the light control layer turns transparent. When the electric field is turned off, the liquid crystal molecules take a random arrangement, and hence, the refraction index of the liquid crystal material greatly deviates from that of the solid polymer. Thus an opaque state is realized, because the light is scattered by the liquid crystal molecules and the light-transmittance becomes low. The device takes advantage of the difference between the transparent state and the opaque state to provide information of various types. In addition to the encapsulated type, dispersion-type liquid crystals include those comprising liquid crystal materials being dispersed in an epoxy resin; those taking advantage of phase separation between the liquid crystal and the resin, which is realized by irradiating a light beam to a mixture of a liquid crystal and a photo-curable resin to cure the resin; and those comprising a three-dimensionally bonded polymer impregnated with a liquid crystal. In the present invention, the term "dispersion-type liquid crystal" encompasses all the types enumerated above.
The above dispersion-type liquid crystal electro-optical devices are free from polarizer sheets and hence have extremely higher light transmittance as compared with those of the conventional electro-optical devices operating in a TN mode, STN mode, etc. More specifically, the transmittance per single polarizer sheet is about 50%. Hence, in an active matrix type electro-optical device using a combination of said polarizer sheets result in a final transmittance of about 1%; in an electro-optical device operating in an STN mode, the actual transmittance is about 20%. Accordingly, much effort in those conventional electro-optical devices is placed to realize a bright display by increasing illuminance of the back-lighting. In contrast to the conventional electro-optical devices, dispersion-type liquid crystal electro-optical devices transmit 50% or more of the incident light. This is a unique superiority of the dispersion-type liquid crystal electro-optical devices which results from their structure free of any polarizer sheets.
As stated in the foregoing, a dispersion-type liquid crystal takes a transparent state and an opaque state, and because it is capable of transmitting a large amount of light, research and development efforts are generally concerned in realizing a transmitting type device. Particularly among them, projection-type liquid crystal devices are the most actively developed types. A projection-type liquid crystal electro-optical device comprises a liquid crystal electro-optical device panel placed in the light path to intervene in the light beam emitted from the light source, so that the light having passed through this panel may be projected on a wall plane through a slit provided at a predetermined angle. The liquid crystal molecules in this panel provide a white opaque state when they are in a random arrangement at a low level electric field below the threshold value at which the liquid crystal molecules do not respond. The light incident to the panel at this instance is scattered upon passing through the panel to greatly widen the light path thereof. Accordingly, the scattered light is mostly cut off by the slit provided subsequent to the panel. A black state realizes on the wall by thus cutting off the scattered light. When an electric field is applied at an intensity over the threshold value, on the other hand, the liquid crystal molecules arrange themselves in response to the electric field to make a parallel arrangement with respect to the direction of the electric field. Thus, the light incident thereto advances straight forward without being scattered to finally realize a bright state with high luminance on the wall.
In the dispersion-type liquid crystal electro-optical devices as described in the foregoing, the contrast of the display depends on the degree of light scattering corresponding to the change in orientation states of the liquid crystal material. Accordingly, it is required that the liquid crystal material is incorporated in the device as numerous minute droplets. The size of the minute droplets should fall in a range of from about 0.05 to 10 .mu.m. In general, they are about 0.3 to 3 .mu.m in size. Such minute liquid crystal droplets can be fabricated under controlled conditions, particularly under strict control of temperature.
If the resin material is solidified at a temperature higher than that at which the liquid crystal droplets precipitate from a mixed system of a liquid crystal material and a resin material, the mixed system cannot undergo sufficient phase separation as to provide the liquid crystal portion and the resin portion. As a result of such insufficient phase separation, the liquid crystal material solidifies in a surrounded state in the resin to give liquid crystal droplets less than 1 .mu.m in size. Such minute liquid crystal droplets do not contribute in light scattering and, moreover, only few droplets can be obtained under such conditions.
If the resin material is solidified at a temperature lower than that at which the liquid crystal droplets precipitate from a mixed system of a liquid crystal material and a resin material, on the other hand, the liquid crystal droplet having precipitated from the mixed system grows into a larger one, or two or more such droplets contact and fuse with each other to give a larger single droplet. The large droplets obtained in this case are too large as to contribute for scattering light. When the resin material and the liquid crystal material are less compatible with each other, the resin material is preferably solidified at a temperature slightly higher than the precipitation temperature of the droplets, because excessively large droplets may result if the resin material is solidified at a temperature lower than the precipitation temperature of the droplets.
On the contrary, the resin material is preferably solidified at a temperature slightly lower than the precipitation temperature of droplets if the resin material is highly compatible with the liquid crystal material, because an insufficient phase separation may result by the resin solidification at a temperature higher than the temperature at which the droplets precipitate. It can be seen from the foregoing that droplets of a pertinent size and at a large number can be obtained only under a controlled temperature condition; when the light-transmitting resin material is solidified at the precipitation temperature of the droplets or at the vicinity thereof.
However, if the precipitation of the droplets occurs at too high a temperature, the resin material should be solidified also at an elevated temperature. Then, problems concerning, for example, increase in fabrication cost of the liquid crystal electro-optical device, handling of substrates, and reproducibility of the process, must be newly confronted. A solution to these problems is to control the temperature of precipitation of the droplets from the mixed system. The precipitation temperature can be controlled by increasing or decreasing the amount of the light-transmitting resin material. However, this method inevitably increases or decreases the amount of the liquid crystal as to impair the light scattering, because light scattering depends on the number of liquid crystal droplets. Moreover, liquid crystal in excess results in too large and non-uniform liquid crystal droplets which impair light scattering and generate a hysteresis.